The formant filters of conventional tone synthesizers may be either of the fixed or sliding type of formant filter. The fixed formant filter has a fixed characteristic with respect to the fundamental frequency of the note being synthesized. Thus the critical frequency (the cut-off frequency for either a low pass or high pass filter) is scaled with respect to the fundamental frequency so as to modify the harmonic content of the tone. In the sliding formant filter the critical frequency which is scaled with respect to the fundamental frequency, is caused to vary with time in response to some input control. The input control may be from the ADSR generator so that the harmonic content of the tone changes from the initial attack through the decay, sustain, and release of the tone. Conventional analog-type tone synthesizers utilize a plurality of filters in the form of resistance-reactance networks to control the frequency characteristics of each tone individually. Where the tones are generated digitally and then converted to analog audio signals, it has been the practice to provide formant filtering action by individually modifying the harmonic coefficients used in synthesizing a waveshape data set using a Fourier transform calculation. Formant filtering in systems of this type are described, for example, in U.S. Pat. No. 3,956,960 and U.S. Pat. No. 4,000,675. In copending application Ser. No. 603,776, filed Aug. 11, 1975, entitled "Polyphonic Tone Synthesizer", now issued as U.S. Pat. No. 4,085,644 there is also described a sliding formant filter which modifies the harmonic coefficients used in synthesizing a set of digitized data representing the amplitudes of sampling points along one cycle of the audio waveform being synthesized. Such known digital systems with formant filtering depend on the individual harmonic coefficients being available in order to control the harmonic content of the synthesized tone.